While nuclear magnetic resonance (NMR) chemical shift spectroscopy has been used for a long time, it is only relatively recently that magnetic resonance spectroscopy (MRS) has included means for localization of the volume from which the signal is received. The localization of the spectroscopic signals facilitates coordinating the MRS with magnetic resonance imaging (MRI) studies. This coordination and combination of studies related to specific volumes of interest enables obtaining a maximum of factual data from particular volumes of a patient thereby enhancing the ability to use the data for diagnostic purposes.
As is well known MRS is similar to MRI in that a relatively strong static magnetic field that has a given direction which is aligned with the Z axis of a cartesian coordinate system, is used. The strong static magnetic field causes the nuclei of certain elements or "spins" to align with and precess about the field. Subsequently radio frequency (RF) pulses of sufficient amplitude and time duration are applied to tip the aligned spins into a transverse plane. The rotational frequency of the RF precession and the frequency of the RF pulse is known as the Larmor frequency which is given by the equation: EQU F=.gamma.Bo/2.pi.
where:
.gamma. is the gyromagnetic ratio, constant for each isotope which exhibits a net magnetic moment; PA1 Bo is the magnetic field strength; and PA1 .pi. is the well known constant 3.1416+
After termination of the RF pulse the rotated spins precess in the transverse plane and also tend to realign with the static magnetic field. The precession of the transverse component in the magnetic field generates transient RF signals also having a Lamor frequency. These signals are known as free induction decay (FID) signals. It is these signals that are used to provide information that is used for spectroscopy and for imaging purposes.
It has been recognised in the prior art that the full potential of NMR spectroscopy in clinical diagnostic applications or in biological in vivo research necessitates the accurate localization of a volume of interest (VOI) within the subject. Without this localization, a meaningful interpretation of MRI signals from a heterogeneous sample is not possible as the signals that originate outside the volume of interest (VOI) would remain undistinguished and would mix with the signals that are meaningful. Thus, it is necessary to discriminate between signals originating within the VOI and signals originating outside the VOI for meaningful spectroscopic data aquisition.
Many different methods for spatial localization of MRI signals are known. Among the methods presently in vogue are the use of surface coils and "depth" pulses. See for example, the articles by M. R. Bendall et al in the Journal of Magnetic Resonance, Vol. 67, p494 (1985) and the article by T. C. Ng et al in Magnetic Resonance Medicine, Vol. 1, p450 (1984). Other methods use:
(1) "static profile gradients", see for example, the article by R. Damadian et al in Science, Vol. 194, p30 (1976);
(2) time dependant gradients, see for example, the article by K. N. Scott et al in the Journal of Magnetic Resonance, Vol. 50, 339 (1982);
(3) static linear gradients in conjunction with selective RF pulses, see for example, the article by R. E. Gordon et al in the Procedings of the Third Annual Meeting of the Society of Magnetic Resonance in Medicine, N.Y. p272, (1984) and the article by W. P. Aue et al in the Journal of Magnetic Resonance, Vol. 56, p350 (1984).
In addition there is a method of selecting the volume of excitation using stimulated echoes written by the inventor herein which appeared in the Journal of Magnetic Resonance, Vol. 70, p488-492 (1986).
Frequency modulated radio frequency pulses have been used in MR imaging experiments as shown, among other places, in an article by D. Kunz which appeared in Magnetic Resonance in Medicine, Vol. 3, pp377-384 (1986).
A more specific explanation of the use of modulated radio frequency pulses and stimulated echo for MRI experiments was covered in an article in Magnetic Resonance in Medicine, Vol. 4, pp129-136 (1987) also by D. Kunz.
Nowhere in the prior art, however, is there any article or teaching of use of frequency modulated RF pulses for spatially localizing spectroscopy data.
Among the problems of the prior art in localization of data obtained for MR spectroscopy are the adverse effects of eddy currents. In the prior art saturation methods are often used in an attempt to minimize the eddy current problems where spins in volumes outside the VOI are saturated and the VOI are left unsaturated.
Another method used by the prior art to overcome the adverse effects of eddy current caused by the high gradient pulses is the use of a sin sinc shaped radio frequency pulse. See for example, the article by D. M. Doddrell et al which appeared in the Journal of Magnetic Resonance, Vol. 70, pp319-326 (1986). However, with both the saturation method and the sin sinc shaped RF pulses, the definition of volume is not good. Also, with the saturation method large volumes have to be saturated, therefore, a large number of saturation pulses are required and because of this, there is a tendency towards a T1 time dependancy, Furthermore, this also increases the RF power depositions in the subject or patient under MRS study. Therefore, the saturation method is not useful under certain circumstances.
Accordingly, there is a need for improved MRS spatial localization methods in obtaining spectroscopic data.
According to a broad aspect of the invention, a magnetic resonant spectroscopic method for accurate localization of a VOI within the subject is provided, said method comprising the steps of:
applying a first set of two 90 degree RF pulses during the application of a first gradient pulse to excite and dephase various unwanted regions in a first direction,
applying a second set of two 90 degree RF pulses during the application of a second gradient pulse to excite and dephase different unwanted regions in a second direction,
applying a third set of two 90 degree RF pulses during the application of a third gradient pulse to excite and dephase different unwanted regions in a third direction, whereby only selected VOI's contain unexcited non-dephased spins,
applying a 90 degree RF excitation pulse to excite spins in said selected VOI's, and
acquiring FID signals from said selected VOI's.
According to a feature of the invention, said sets of 90 degree RF pulses are sets of frequency modulated 90 degree RF pulses.
According to yet another feature of the present invention the last excitation pulse is also a frequency modulated pulse. However, in accordance with the invention the last excitation pulse may be an amplitude modulated pulse.
According to yet another feature of the present invention there is a delay time between the last set of 90 degree RF pulses and the last 90 degree RF pulse.
According to yet another feature of the present invention means are provided for overcoming the eddy current effects and for providing volumes of interest that have efficiently and effectively defined boundaries and are not plagued by T2 or T1 time dependencies and therefore, are useful for collecting data even when the time period T2 is especially short.
A further related feature of the invention is the provision of obtaining more clearly defined limits on the VOI for the same bandwidth. Furthermore, with the present invention the number of saturation pulses required for defining the VOI are cut in half.
The present invention provides for low power deposition, reduces the T1 dependancy and provides a reduced peak power compared to amplitude modulation spatial localization methods for MRI.